1. Field of the Invention
The present invention relates to the field of nanocrystals and to a method of processing and resulting compositions of same.
2. Related Art
The world of “Nanotechnology” has been simultaneously heralded by some who view its advances as providing the next great technological evolution, and derided by others who view it as just the latest buzz-word technology to attract venture capital investment. While their fundamental views on the promise of the technology are at odds, members of both camps will point to a number of common issues that nanotechnology must address if it is ever to fulfill its promises.
Of particular note is that, while both camps tend to acknowledge that nanomaterials often have unique and potentially valuable properties, e.g., structural, electrical, opto-electrical and thermoelectric, the ability of scientists and eventually users or consumers to access these unique and valuable properties can present a substantial hurdle to realizing the full benefits of these materials.
For example, nanocrystals have gained a great deal of attention for their interesting and novel properties in electrical, chemical, optical and other applications. Such nanomaterials have a wide variety of expected and actual applications, including use as semiconductors for nanoscale electronics, optoelectronic applications in emissive devices, e.g., nanolasers, LEDs, etc., photovoltaics, and sensor applications, e.g., as nanoChemFETS.
While commercial applications of the molecular, physical, chemical and optical properties of nanocrystals are beginning to be realized, it has been difficult to fully capitalize on the unique properties of nanocrystals because of the difficulties related to their preparation and processing. In order to incorporate the nanocrystals into devices, the nanocrystals must be further processed from the batch reaction mixtures. The batch reaction mixtures contain by-products, impurities, excess surfactant and other matter that must be separated from the nanocrystals during processing.
Methods of processing the nanocrystals traditionally have been based on the solubility differences between nanocrystals, the surfactants and other impurities or reaction by-products. Traditionally, solvents are added to the batch reaction mixture to cause the nanocrystals to precipitate, thus allowing for their isolation by filtration or centrifugation. The isolated nanocrystals are then redispersed in an appropriate solvent and the precipitation is repeated any number of times until the appropriate level of purity is obtained. When the solubility of the nanocrystals and the surfactant or other impurities are similar in a given solvent mixture, however, the process must be repeated a greater number of times to purify the nanocrystals. This increases the cost and difficulties associated with processing the nanocrystals.
It is unclear from the current state of the art whether other methods are preferable over the traditional precipitation methods. For example, Khitrov, G. A. and Strouse, G. F. J. Am. Chem. Soc. 125:10465-10469 (2003) describe a method of characterizing ZnS nanomaterials using MALDI-TOF mass spectrometry and teach that chromatographic methods of characterizing nanomaterials suffer from limitations such as sample retention. Also, Akcakir, O. in Silicon NanoCrystal Characterization by Fluorescence Correlation Spectroscopy (2001) (Ph.D. dissertation, University of Illinois), available at http://www.physics.uiuc.edu/Research/Publications/theses/copies/akcakir/chapter5.pdf. Akcakir teaches traditional techniques such as chromatography are not readily applicable to the study of nanomaterials. Ackakir teaches that because the nanoparticles are often compatible with both the liquid and solid phases characterization of sample quality may be difficult. Additionally, Krueger, K. M. teach that size exclusion chromatography of CdSe dots does not seem feasible (see, e.g., Krueger, K. M., Comments on CdSe Nanocrystal Research, available at http://nanonet.rice.edu/research/karl_res.html (Apr. 27, 2004)).
Despite the teachings of the above references, there has been limited success in the analytical characterization of certain types of nanomaterials using techniques such as liquid chromatography. For example, one group has shown that high performance liquid chromatography (HPLC) can be used to characterize size distributions of metallic nanoclusters (see, for example, Wilcoxon, J. P., et al., Nanostruct. Mater. 9:85-88 (1997); Wilcoxon, J. P. et al. Langmuir 16:9912-9920 (2000); and Wilcoxon, J. P. et al. J. Chem. Phys. 115:998-1008 (2001)). In one study, Wilcoxon et al. teach the eluent must be spiked with dodecane thiol, a ligand for the nanoclusters, to eliminate chemical interactions with the column (see Wilcoxon 2001 at 1000). This spiking of the eluent with ligand limits the utility of this method in processing nanomaterials because excess ligand is not desirable for most device applications. In addition, the ligand and nanomaterial (dodecane thiol and gold nanocluster) are readily separated using the traditional precipitation technique (see Wilcoxon 2000 at 9917).
Fischer, Ch.-H. et al. Ber. Bunsenges. Phys. Chem. 93:61-64 (1989) describe the fractionation of a colloid of CdS particles using size exclusion chromatography. However, in this study, the eluent contained both Cd(ClO4)2 and sodium hexametaphosphate as stabilizer for the particles. While this method was useful in characterizing size distributions of the particles, the method has almost no utility in purifying and processing nanomaterials. The eluent contains contaminants that would interfere with device fabrication and operation. In addition, the particles studied in Fischer are insoluble colloids.
Korgel, B. and Pell, L., Proceedings of SPIE 4808:91-98 (2002) teach a method of synthesizing and characterizing silicon nanocrystals. Korgel teaches the use of size exclusion chromatography to purify analytical quantities of nanocrystals from by-products with moderate size separation. However, in this method, Korgel does not teach the use of chromatography to remove excess ligand or surfactant. Furthermore, the ligand/surfactant, 1-octane thiol, is readily separated from the reaction mixture using traditional precipitation techniques. In addition, the method of Korgel is performed in air, which would not be applicable to air sensitive nanomaterials.
Accordingly, it would be desirable to have a method of processing a variety of different types of nanocrystals that is not based on their precipitation from solvents. Also, it would be desirable to have a method of processing nanocrystals that separates similarly soluble surfactants and ligands from the nanocrystals. Furthermore, some soluble nanocrystal populations cannot be made to precipitate using standard methods and solvents. Precipitation methods are inadequate for the purification of such nanocrystals.